This invention is in the field of integrated optical devices. The invention relates to apparatus for coupling light into channel waveguides. The invention relates particularly to integrated optical devices which incorporate one or more lenses situated to efficiently couple light into a channel waveguide. The invention has particular application to arrayed waveguide gratings but may be used in other optical devices. Another aspect of the invention relates to lenses for use in integrated optical circuits.
Integrated optical devices of various kinds can be made by combining optical elements, such as waveguides, lenses, arrayed waveguide gratings and others. Such devices are typically formed within an optical layer on a generally flat substrate and hence are described generically as planar lightwave circuits (PLCs).
PLCs typically comprise various combinations of planar waveguides and/or channel waveguides. Such waveguides are described by H. Kogelnik, xe2x80x9cTheory of Optical Waveguides,xe2x80x9d in Guided-Wave Optoelectonics T. Tamir ed., Springer-Verlag, Berlin, 1988, and also by H. Nishihara, M Haruna, and T Suhara, Optical Integrated Circuits, McGraw Hill, New York, 1987.
In a slab waveguide (sometimes referred to as a planar waveguide), light is restricted to propagate in a region that is thin (typically between 3 xcexcm and 10 xcexcm) in one dimension, referred to herein as the lateral dimension, and extended (typically between 1 mm and 100 mm) in the other two dimensions. A plane that is perpendicular to the lateral dimension of the PLC is defined as the plane of the PLC. The longitudinal direction is defined as the direction of propagation of light at any point on the PLC; the lateral direction is defined to be perpendicular to the plane of the PLC; the transverse direction is defined to be perpendicular to both the longitudinal and the lateral directions.
Light propagating along a channel waveguide in a PLC has an optical field that is substantially confined in both the lateral direction and the transverse direction. In a typical channel waveguide, the field is substantially confined within a region that extends between 3 xcexcm and 10 xcexcm in the lateral direction, herein referred to as the height, and extends between 3 xcexcm and 100 xcexcm in the transverse direction, herein referred to as the width.
Slab waveguides may have various constructions. Constructions which use doped-silica are usually preferred because such constructions have attractive properties including low cost, low loss, low birefringence, stability, and compatibility for coupling to fiber. A typical doped-silica slab waveguide comprises a core layer of silica glass lying between top and bottom cladding layers of silica glass. The layers are doped so that the core layer has a higher index of refraction than either the top or bottom cladding layers. When layers of silica glass are used for the optical layers, the layers are typically deposited on a silicon wafer.
Slab waveguides may also be made using materials other than silica glass. For example, a slab waveguide may comprise three or more layers of InGaAsP. In this example, adjacent layers have compositions with different percentages of the constituent elements In, P, Ga, and As. Slab waveguides may also be made using layers of optically transparent polymers or a layer of a material having a graded index such that the region of highest index of refraction is bounded by regions of lower indices of refraction.
Channel waveguides may have constructions similar to slab waveguides with the addition of side cladding material that limits the transverse extent of the waveguide. The side cladding material has an index of refraction that is lower than that of the core material. The side cladding layer and top cladding layer are usually made of the same material.
There are various optical devices in which light propagating in a free propagation region, such as a slab waveguide, is coupled into one or more channel waveguides. An example of such a device is an arrayed waveguide grating router (AWGR). In a typical AWGR, light from one or more input ports is coupled into a first slab waveguide. The first slab waveguide is, in turn, coupled to a second slab waveguide by an arrayed waveguide grating (AWG). Light propagates through the second slab waveguide. The second slab waveguide is coupled to at least one output port.
An AWG comprises a plurality of channel waveguides. The length of the ith waveguide in the AWG is denoted as Li. The angular dispersion that is provided by the AWG is determined in part by the difference in length between adjacent waveguides, Li+1xe2x88x92Li. Details of construction and operation of AWGRs are described in M. K. Smit and C. Van Dam, PHASAR-Based WDM-Devices: Principles, Design, and Application, IEEE Journal of Selected Topics in Quantum Electronics, vol. 2, no. 2, pp. 236-250 (1996); K. McGreer, Arrayed Waveguide Gratings for Wavelength Routing, IEEE Communication Magazine, vol. 36, no. 12, pp. 62-68 (1998); and K. Okamoto, Fundamentals of Optical Waveguides, pp. 346-381, Academic Press, San Diego, Calif., USA (2000). Each of the publications and patents referred to in this application is herein incorporated by reference in its entirety. The particular output port to which light entering the AWGR at a particular input port is most strongly coupled is wavelength-dependent. Possible applications of AWGRs include, but are not limited to demultiplexing, multiplexing, or providing Nxc3x97N routing.
An important consideration in the design of PLC devices, such as AWGRs, is that the insertion loss provided by the device should be small. It is a challenge to realize an AWGR with an insertion loss that is less than an insertion loss specification for a particular application. Many factors contribute to insertion loss including:
coupling losses where optical fibers are coupled to the PLC;
losses associated with the diffraction of light into diffraction orders that are not coupled to any output channel waveguide;
coupling losses between the AWG region and the input slab waveguide; and,
coupling losses between the AWG region and the output slab waveguide.
In certain devices it is also very important to minimize any back-reflection of light.
Dragone, U.S. Pat. No. 5,002,350, discloses that the coupling loss between a slab waveguide region and an AWG region may be reduced by providing adjacent channel waveguides of the AWG with a separation that is sufficiently large to substantially prevent mutual optical coupling in at least one region of the AWG and by making the separation between adjacent channel waveguides of the AWG sufficiently small to provide substantial mutual optical coupling in the region where the channel waveguides are coupled to the slab waveguide. In this region, it is preferred that adjacent channel waveguides have a separation that is as small as possible within the limits imposed by the fabrication process.
Li, U.S. Pat. No. 5,745,618, discloses another construction for reducing the coupling loss between a slab waveguide region and an AWG region. Li provides a transition region between the grating region and the slab waveguide region. The transition region comprises silica paths that traverse the waveguides of the grating. By arranging the silica paths to have widths that increase as the slab region is approached, the mode transition is made more gradual. This can reduce transition loss.
AWGRs having vertically tapered waveguides are disclosed in A. Sugita, A Kaneko, K. Qkamoto, M. Itoh, A. Himeno, and Y. Ohmori, xe2x80x9cVery low insersion loss arrayed-waveguide grating with vertically tapered waveguide,xe2x80x9d IEEE Photon. Technol. Lett. Vol. 12, no. 9, Pp. 1180-1182 (2000) and J. C. Chen, and C. Dragone, xe2x80x9cA proposed design for ultra-low loss waveguide grating routers,xe2x80x9d IEEE Photon. Technol. Lett., Vol. 10, no., Pp. 379-381 (1998).
Despite the extensive research that has been carried out to date with a view to reducing the insertion loss of PLCs, there remains a need for alternative ways to construct devices, such as AWGRs, which have reduced insertion loss.
The prior art discloses various constructions for lenses which may be incorporated in PLCs.
Bhagavatula, U.S. Pat. Nos. 5,253,319 and 5,612,171 disclose lenses which may be integrated into planar optical waveguides. The lenses each comprise an interface adjacent a cavity. The cavity is either empty (i.e. has an index of refraction that is substantially equal to 1.0) or is filled with a material having an index of refraction that is substantially higher than the index of refraction of the core material in the optical waveguide exterior to the lens. Bhagavatula teaches that it is desirable to maintain a substantial difference between the refractive indices of the cavity and the adjacent core regions of the optical waveguide.
Bhagavatula discloses various devices which use such lenses including an 1xc3x97N optical coupler and a Mxc3x97N optical coupler. The 1xc3x97N coupler comprises a slab waveguide with one input channel waveguide optically coupled to one side of the slab waveguide and N output channel waveguides optically coupled to the opposite side of the slab waveguide. The boundary of the lens that is proximal to the input channel waveguide comprises one curved arc configured to collimate the light that emerges from the input channel waveguide. The boundary of the lens that is proximal to the output channel waveguides comprises N arcs; each arc is configured to focus the collimated light into its respective output channel waveguide. In general, the Mxc3x97N couplers described in U.S. Pat. Nos. 5,253,319 and 5,612,171 have lenses configured so that, according to geometric optics, the light that emerges from each input channel waveguide forms N images and each image location corresponds to the location of one of the output channel waveguides.
The literature describes various devices which include a plurality of integrated optical lenses arranged to suit a particular application. These include:
Schwering et al., U.S. Pat. No. 5,528,717, which discloses a beam waveguide for millimeter and sub-millimeter wave regions which comprises a plurality of integrated cylindrical lenses spaced apart along the bean waveguide; and,
Fournier et al., U.S. Pat. No. 5,210,801, discloses various optical devices, some of which incorporate a plurality of integrated lenses.
The literature describes various lens constructions and methods for fabricating lenses in integrated optical devices. The literature includes:
M. M. Minot and C. C. Lee, xe2x80x9cA new guided-wave lens structure,xe2x80x9d J. Lightwave Technol., vol. 8, no. 12, Pp. 1856-1862, (1990) which discloses a lens formed with a semiconductor waveguide exterior to the lens and a different semiconductor waveguide interior to the lens;
Aagard, U.S. Pat. No. 4,141,621, which discloses a plasma-etching process for producing integrated lenses;
Stoll, U.S. Pat. No. 4,755,014, which discloses another technique for making integrated lenses;
Gidon et al. U.S. Pat. No. 4,865,453 which discloses an integrated optical displacement transducer which includes a lens formed by reactive ion etching a silica substrate to provide an air-filled cavity; and,
Spillman, U.S. Pat. No. 4,547,262 which discloses an integrated optical lens formed by proton exchange.
Notwithstanding the above mentioned techniques, there is a need for optical devices such as AWGRs which have reduced insertion loss. There is a need for an array of planar optical lenses that are easily fabricated and suitable for coupling light between an arrayed waveguide grating (AWG) and a slab waveguide.
This invention provides optical matching elements useful for combining or separating optical signals. The optical matching elements may be used for coupling light between a slab waveguide and an array of single waveguides in devices such as AWGs. Devices according to the preferred embodiments of the invention comprise an optical lens array either between the slab waveguide area and array waveguide region or between the slab waveguide region and output waveguide. The lenses may be concave (where the effective index of the lens interior is smaller than that of a region surrounding the lens) or convex (where the effective index of the lens interior is larger than that of a region surrounding the lens). The index contrast is the difference between the index of refraction (or effective index) of the lens interior and the index of refraction (or effective index) of the material(s) surrounding the lens.
In preferred embodiments of the invention the optical coupling element provides a beam having a waist of a width that provides a good mode match with the fundamental mode of the channel waveguide associated with the optical coupling element. Low index contrast lenses are preferred over high index contrast lenses for this application because high index contrast lenses reflect a larger portion of the light. This has an adverse affect on the coupling efficiency and furthermore, the reflected light may be problematic.
One aspect of the invention provides an optical apparatus comprising: a first slab waveguide; a first plurality of channel waveguides optically coupled to the first slab waveguide; and an optical matching element associated with each of the plurality of channel waveguides. The optical matching element comprises at least one lens formed in the first slab waveguide and located to couple light between the first slab waveguide and the channel waveguide.
In preferred embodiments the first slab waveguide comprises a core layer having a first index of refraction, the lens comprises an interior region having a second index of refraction, and the first index of refraction differs from the second index of refraction by less than 0.04.
Another aspect of the invention comprises a planar lightwave circuit comprising an integrated low index-contrast lens. In this specification and the appended claims the term xe2x80x9cplanar lightwave circuitxe2x80x9d refers to a type of lightwave circuit construction and does not require mathematical planarity of the circuit or any part of it. The lens comprises an interior, and a boundary separating the interior of the lens from an exterior region of the planar lightwave circuit. The interior has a first effective index and the exterior has a second effective index. The first and second effective indices differ from one another by an amount not exceeding 0.04.
Another aspect of the invention provides an integrated optical coupler. The optical coupler comprises a first slab waveguide, at least one input port optically coupled to the first slab waveguide on a first side, a plurality of output waveguides optically coupled to the first slab waveguide on a second side opposed to the first side, and at least two integrated optical lenses disposed within the first slab waveguide. Each of the lenses is at a location adjacent to a mouth of a different corresponding one of the output waveguides. Each of the at least two lenses comprises an interior, and a boundary separating the interior of the lens from a surrounding region of the first slab waveguide. The interior has a first effective index and the surrounding region has a second effective index. The first and second effective indices differ from one another by an amount not exceeding 0.04.
In preferred embodiments each of the lenses is spaced apart from a mouth of its corresponding channel waveguide by a distance which is less than twice a focal length of the lens. The distance is preferably less than 20 xcexcm.
The output waveguides may comprise tapered regions proximate the first slab waveguide wherein the tapered regions decrease in width with distance from the first slab waveguide.
A further aspect of the invention provides an arrayed waveguide grating router comprising: first and second slab waveguides optically coupled by an arrayed waveguide grating comprising a plurality of AWG channel waveguides; one or more first I/O channel waveguides optically coupled to the first slab waveguide; one or more second I/O channel waveguides optically coupled to the second slab waveguide; and at least one optical matching element in the first slab waveguide. The optical matching element is located in an optical path which extends through the arrayed waveguide grating router between one of the first I/O channel waveguides and one of the second I/O channel waveguides. The optical matching element comprises an optical lens integrated with the first slab waveguide.
In preferred embodiments the optical matching element comprises a train of lenses. Each of a plurality of the lenses in the train has a thickness of 20 xcexcm or less.
Yet another aspect of the invention provides a method for providing a lens integrated in a waveguide comprising a core layer and top and bottom cladding layers. The method comprises irradiating with ultraviolet light a lens-shaped portion of the core layer of the waveguide. The method preferably includes providing a mask defining a lens shape, and irradiating with ultraviolet light a lens-shaped portion of the core layer defined by the mask. A variation of this method may be used for providing an optical matching element in a waveguide comprising a core layer and top and bottom cladding layers. The method comprises providing a mask defining a lens shape, and, changing an effective index of the core layer in a plurality of adjacent lens-shaped regions by irradiating with ultraviolet light areas of the core layer defined by the mask to provide a train of closely spaced lenses integrated in the core layer of the waveguide.
Further details, features and advantages of the invention are discussed below.